1. Field of the Invention
This invention relates to the field of testing the electrical integrity of electrical pathways on a substrate, and more specifically, to a contactless method and apparatus for testing the interconnect networks of, for example, multichip modules, flat panel displays, printed circuit boards, and other substrates for opens and shorts.
2. Background Information
In the manufacture of semiconductor devices, such as multichip modules (MCMs), flat panel displays, printed circuit boards (PCBs), and other substrates, advances in integrated circuit technology are resulting in increased chip packing densities which require finer linewidths and an increased number of substrate interconnection networks. Complex substrates contain thousands of interconnect wiring networks with more than ten thousand chip interconnect bonding pads (pads) to interconnect the integrated circuit chips. As the number of interconnect networks (networks) increase on a substrate, so do the number of defects. Therefore, testing and inspection of these networks for opens and shorts becomes increasingly more important.
Testing and inspection of interconnect networks are important requirements that reduce the cost and increase the reliability of substrates. Existing methods and apparati for testing the substrate have made it economically advantageous to test the substrates before bonding expensive chips to them. Such methods and apparati ultimately increase the final test yield and reduce expensive rework. Examples of such methods and apparati are the Mechanical Probe and Voltage Contrast Systems.
The Mechanical Probe System
Mechanical Probe systems are generally categorized into two types: the single probe system and the dual probe system. Both systems work by contacting the probes to a pad on an interconnect network. The single probe system measures the capacitance of an interconnect line. The measured value of capacitance is then compared to a reference or known capacitance value. If the measured value equals the known value then the pads are properly interconnected. If the measured value is greater than the known value then a short is detected. If the measured value is less than the known value then an open is detected.
The dual probe system works by taking two probes, placing them on separate pads, and measuring the resistance between those pads. The measured value of resistance is then compared to a reference or known resistance value. If the measured value equals the known value then the pads are properly interconnected. If the measured value is different than the known value then either an open or a short is detected.
Mechanical probe systems are effective but inefficient. The mechanical motion required to move and place a probe on a pad is slow and decreases the throughput of the testing system. Mechanical probes also have other disadvantages. Due to the actual physical contact between the probe and pad in a mechanical probe system, the probe may damage the pad or leave particulate contamination on the pad surface. Pad damage and contamination ultimately decrease the test yield and have a significant impact on device performance. Another disadvantage of mechanical probes is that they are not applicable to features below 25 microns. As semiconductor device feature sizes become smaller and smaller, mechanical probes become more and more obsolete.
Electron Beam Substrate Testers (EBST).
In response to the problems encountered with the mechanical probe system, contactless systems for testing substrates, also known as electron beam substrate testers (EBST), have been developed. Electron beam substrate testers use a focused beam of electrons (e-beam) to interrogate the pads of the interconnect networks. EBST systems function in substantially the same manner as the single probe mechanical system. The e-beam is used to apply a charge to a network pad (charged pad) and to measure the presence or absence of charge on other pads, thereby testing for network open circuits and network to network shorts, as will be described in more detail below.
There are several advantages of EBST systems over mechanical probe systems. The electron beam is an essentially non-contact probe which does not damage the pads and circuitry of the device under test (DUT), thus increasing the final test yield of DUTs. Also, the e-beam can be vectored from pad to pad in microseconds while a conventional mechanical probe requires milliseconds per move. The speed advantage of the e-beam is compounded by a further reduction in the number of test vectors. In order to achieve 100% fault detection using an EBST system on a substrate with N pads, f(N) test vectors are required. By comparison, the dual probe mechanical systems described above require f(N.sup.2) test vectors. Single probe mechanical systems, like EBST, require f(N) test vectors but are 1000 times slower than EBST and have limited probing resolution. For example, a complex substrate can be tested, using an EBST system, in less than two minutes including handling time. A further advantage of EBST systems is that the e-beam can be focused to a finer resolution such that the e-beam is defined to be smaller in diameter than the pad being charged or interrogated. Thus, EBST systems are applicable to features well below 25 microns.
EBST systems operate using the secondary emission yield. The secondary emission yield (.delta.) is the ratio of the number of secondary electrons emitted from the pad (secondary electrons, m) to the number of primary electrons landing on the pad (primary electrons,n), .delta.=m/n. FIG. 1 illustrates the secondary emission yield plotted as a function of the energy of the primary e-beam landing on the pad. Although the value of .delta. varies from material to material the general shape of the graph is the same. Initially, the value of .delta. is below 1, since the energy of the primary e-beam is insufficient to dislodge a large number of secondary electrons. As the energy of the primary e-beam increases, the value of .delta. increases. However, as the energy of the primary e-beam increases further, the primary electrons penetrate deeper into the pad and although more secondary electrons are dislodged fewer reach the surface of the pad, so that the value of .delta. starts to decrease.
In one example of an EBST system, a substrate, which contains a complex network of interconnect lines and pads, is floating with respect to ground. In other words, the substrate is insulated while being bombarded by the e-beam. Located in the vicinity of the substrate is a collector which is held sufficiently positive to attract electrons. In general, this collector is referred to as an extract grid located above the substrate and is held at approximately +10 volts. When a primary e-beam lands on a pad of the substrate with an energy, such that .delta.&lt;1, the pad will acquire more electrons than it loses and will eventually be negatively charged. When the e-beam lands with an energy, such that .delta.&gt;1, the secondary electrons emitted by the pad are collected by the extract grid and the pad will eventually be positively charged. When .delta.=1, the pad acquires no net charge.
An EBST system uses the secondary emission spectrum detected at a network pad to determine if there is a charge on that pad. FIG. 2 illustrates a secondary electron spectrum of a preferred embodiment of the present invention. As an example, the secondary electron spectrum in FIG. 2 demonstrates the difference in intensity of the secondary emitted electrons coming from a charged and uncharged pad. FIG. 2 illustrates, as an example, that when a pad is uncharged its secondary electron spectrum has a peak centered near 5 eV. When a pad is charged the secondary electron peak shifts over by an amount approximately proportional to the amount of charge on the pad.
The Voltage Contrast System
The voltage contrast system essentially measures the presence or absence of charge on an interconnect line in an analogous fashion to that of the mechanical probe testing system. FIG. 3 illustrates how a typical voltage contrast system works on a substrate 300 that contains several interconnect networks. Interconnect networks (networks) are a series of wiring networks with bonding pads (pads) that connect the circuitry of VLSI chips. Illustrated in FIG. 3 are three networks, a first network containing network pads 310 and 311 (pads), a second network containing pad 312, and a third network containing pad 313. An electron beam 350 (e-beam) is positioned on a pad 310. First, the voltage of pad 310 is measured for a precharged condition. Voltages are measured by a secondary electron energy analyzer (FIG. 4) to detect corresponding changes in the secondary electron distribution. The type of change in the secondary electron distribution will tell whether the pad is charged or uncharged (as is described in the discussion of FIG. 2 above). If a charge is present on pad 310 then a short to a previously charged network is assumed since the network containing pad 310 has not previously been interrogated and, therefore, no charge should be present on pad 310. If there is no preexisting charge, the second step of the system is to charge pad 310 to a predetermined voltage using e-beam 350. The third step is to determine the continuity of the network containing pad 310. This is accomplished by probing the remaining pads (311, 312, and 313) on substrate 300 for the predetermined voltage. For example, since pad 311 is on the same network as pad 310, then a charge should be found on pad 311. If a charge is found on pad 311 then pad 310 and pad 311 are properly connected. If no charge is present on pad 311 then an open in the network is found. Since pads 312 and 313 are not contained in the same network as pads 310 and 311 no charge should be present on pads 312 and 313. If a charge is found on pads 312 and 313 then a short to a previously tested network is assumed since neither network containing pads 312 and 313 have been previously tested. These three steps are repeated for each network on the substrate.
Before testing the substrate and between major test sequences involving numerous networks, electrostatic charges must be removed from the substrate being tested in order to avoid testing errors. Since it is impossible to interrogate a pad without depositing some amount of charge on a pad the amount of electrostatic charge that is built up on a substrate during a test sequence can be very substantial. This build up of electrostatic charge on the substrate can make testing the substrate more difficult and cause testing errors. Thus, to avoid these problems, an electron flood gun (flood gun) is used to remove the electrostatic charges on a substrate. The flood gun floods the entire substrate with electrons effectively erasing any charge disparity between the networks on the substrate.
FIG. 4 illustrates a secondary electron analyzer as used in a voltage contrast system. The substrate 400 is located on x-y stage 440 (stage) and is floating with respect to ground. E-beam 450 electronically interrogates substrate 400 one section at a time. Substrate 400, for example, can be divided into several one inch by one inch sections. If substrate 400 is larger than the section e-beam 450 is able to interrogate, then stage 440 must mechanically move substrate 400 such that every network on substrate 400 may be interrogated. Extract grid 410 (grid) is held at approximately +10 volts and acts as a collector. Detector 420 is generally a photomultiplier tube (PMT) and detects the changes in the secondary electron distribution of a network pad. Electron flood gun 430 (flood gun) floods the entire substrate with electrons as described above.
The voltage contrast system while an improvement over the mechanical probe system, in that it is a contactless system, still has several disadvantages. Although the e-beam may be vectored 1000 times faster than mechanical probes, the voltage contrast system is still relatively slow for large substrates. In the voltage contrast system, once a charge is placed on a pad the e-beam must interrogate every other pad on the substrate to test for the presence or absence of that charge. In complex substrates the number of pads is substantial and every pad must be tested. This means that for relatively large substrates, for example an 8 inch by 8 inch substrate, the x-y stage must zig and zag back and forth so that every pad on the substrate can be interrogated.
Another problem which decreases throughput of the voltage contrast system is the need to remove electrostatic charges from the substrate in order to avoid testing errors. Flood guns cause substantial noise at the PMT and the testing must wait for several milliseconds after flood to be sure the electron cloud has dissipated before testing can resume. Since this procedure must be repeated thousands of times during the testing of complex substrates, the testing time is substantially prolonged.
An additional disadvantage particularly sesitive with the voltage contrast system is that testing errors can result from beam spillover. Beam spillover occurs when two or more networks are adjacent to each other, for example 100 microns apart, and electrons from the e-beam interrogating one pad spill over to a pad on an adjacent network. Beam spillover, in this instance, gives erroneous results when the voltage on the pad of the adjacent network is later interrogated. Such beam spillover would lead the tester to erroneously conclude that the pad on the adjacent network is shorted to a previously tested network.
With electron beam testing one must ascertain proper registration. Improper registration occurs when the e-beam does not land directly on the pad and can occur if the e-beam has a low resolution or is improperly vectored. If improper registration occurs the e-beam deposits a charge on the insulator. If a relatively high voltage is used, for example 10 keV, as is commonly used with the voltage contrast method, the insulator will hold this charge indefinitely with very little leakage. The charged insulator causes a distortion of the field and the tester can no longer function with high reliability.
There are several ways to avoid the deleterious effects of improper registration. The first, and most obvious, is to vector the e-beam to the exact location of the pad. This requires registration of the substrate with known fiducial marks. Fiducial marks are marks on the substrate that allow the substrate to be aligned with the testing system such that the e-beam may be vectored to the registered interconnect networks and pads using an x-y coordinate system. The placement of fiducial marks on the substrate requires additional processing steps and equipment. The additional processing steps and equipment require substrate manufacturers to make costly changes in their process which most manufacturers are reluctant to do.
A second way to avoid the deleterious effects of improper registration is to use two different voltages to accelerate the electron beam. The first beam would be used to find the exact location of the pad being interrogated. The first beam would have a low voltage, approximately 600 to 1000 volts, such that no deleterious effects would be caused if the beam landed on the insulator. Once the exact location of the pad is found with the first beam, the second beam, which has a higher voltage, generally 10 keV, can then be vectored to that exact location. This allows interrogation of the pad without the effects of improper registration. However, using two beams is very costly.
Another method to avoid the deleterious effects of improper registration and still obtain the necessary beam resolution is to accelerate the beam at approximately 5 to 10 keV and then decelerate the beam to approximately 600 to 1000 volts just before it lands on the pad. This method requires some special equipment to be incorporated in the tester but does not significantly decrease the throughput of the tester. However, the deceleration of the electron beam tends to deteriorate the quality of the spot size and limits the area which can be addressed electronically.
Thus, what is needed is a method and apparatus to test and inspect the interconnect networks of MCMs, PCBs, flat panel displays, and other substrates for opens and shorts that will increase the test yield and increase the throughput of the system as technology advances and the density of interconnect networks on substrates increase.